The present invention relates to the making of filled bags in vertical form, fill and seal machines, and is more particularly concerned with alleviating the stretch stress at the thermo-cross-seals at the upper ends of filled bags.
In the production of filled bags in a form, fill and seal machine, thermoplastic bag making film is tubularly shaped about a forming and filling tube and drawn downwardly by bag length sections by means of a thermo-cross-sealing mechanism. Such mechanism customarily comprises cooperative reciprocating bars which function to close below the discharge end of the forming and filling tube to seal the top edge of each bag after it is filled, while at the same time sealing the lower edge of the succeeding bag. Then while still gripping the cross-seal, the closed bars pull the bags and the bag making material downwardly a bag length section and release the now sealed and filled bag which has been detached from the next succeeding bag by means associated with the sealing and pull down mechanism. By way of example, U.S. Pat. No. 4,829,745 is referred to as showing a typical machine of the kind described. To any extent necessary, that patent is incorporated herein to lessen the need for a more detailed description of elements environmental to the present invention.
A problem that has been encountered is that since the cross-sealing temperature is such as to initiate fusion, there is a tendency for sagging or stretch stress due to the downward pull from the weight of the filled bag upon the cross-seal at the upper edge of the filled bag. This is particularly true where fairly heavy loads are imposed in the bag contents. The cross-seal is especially vulnerable to stretch damage where the bag making material is equipped with extruded plastic profile reclosable zipper means, and during the cross-sealing there is of necessity fusion at the spot sealing of the substantially greater mass of the zipper profiles as compared to the cross-sectional mass of the bag making film itself.